Composite wood products, such as board, may be formed by consolidating a loose mat of lignocellulosic materials under heat and pressure, until the individual lignocellulosic elements adhere together to form a solid wood-like product. The lignocellulosic materials may take the form of wood materials, such as, particles, chips, fibers and/or the like and it will be understood that these terms are used interchangeably herein. Typically, the materials forming the mat are treated with a binder, such as a resin, before heat and consolidation are applied, to enhance adherence of the materials and improve the resulting properties of the finished product.
Consolidation of the mat is generally conducted in a press. A conventional press for consolidating a binder treated wood composite mat to a particular molded shape, such as, for example, a board, includes two opposing press platens spaced to define a mold cavity. At least one platen is heated through conduction, such as through the use of electric heating coils or by passing a heated fluid or gas medium, such as steam, through conduits heating coils or by passing a heated fluid or gas medium, such as steam, through conduits located in the platen body. Upon contact with the mat, heat is transferred from the platen to the mat by conduction. This process is known as hot pressing.
Urea formaldehyde (UF) resin or isocyanate (MDI) resin have typically been the binder of choice in hot pressing of composite wood products due to lower curing temperatures, reasonably short press cycles and superior properties imparted to the finished product in the short press cycles. Recently, due to significantly lower cost in use, attention has been directed to methods using phenol formaldehyde (PF) resins. However, the properties of composite products hot pressed with PF resins are inferior to those made with UF or MDI resins, and the press time for PF resins is typically found to be significantly longer.
Thus, it is known that certain resins having, for example, rapid curing rates or high curing temperatures, yield composite wood products with inferior properties when produced in a conventional press by hot pressing. U.S. Pat. No. 4,850,849 to Hsu et al. discloses that prior art presses are not capable of producing sufficiently high temperatures within a reasonable time frame to achieve curing of binders such as phenol formaldehyde resin. Additionally, it is believed that the slow transfer of heat by conduction from a conventional platen to a mat, particularly a thick mat, causes temperature differentials across the thickness of the mat. The temperature differentials may cause, for example, resin and fibers at or near the surface of the mat adjacent to the heated platen to be exposed to excessive heat, while materials at the core of the mat may be exposed to insufficient heat. The temperature differential across the thickness of a mat during curing in a conventional press can thus lead to over-curing and/or under-curing of portions of the thickness of the mat, resulting in structural and/or aesthetic flaws in the finished product. Resins with rapid curing rates or high curing temperatures are particularly susceptible to the negative effect on resin curing of temperature differentials across the thickness of the mat. For the foregoing reasons, phenol formaldehyde resins generally have been considered unsuitable for producing thick composite board products in a conventional press.
Also, although conventional presses have been successful in making fiberboard products using only conduction heat (hot pressing), today's manufacturing demands require faster cycle times on the press and the use of stronger high-temperature resins to produce highly detailed, higher density, and thicker fiberboard products. It is known that some of the disadvantages of conventional platens can be overcome by supplying, or injecting, steam directly into a mat through modified press platens provided with steam injection ports for that purpose. This is generally known as "steam injection" pressing. The steam passes from the injection ports into interstitial spaces between the wood particles, chips and/or fibers forming the mat, thus carrying heat quickly and uniformly to the core of the mat. Steam injection pressing has several advantages. Steam injection pressing speeds the curing of typically dimensioned boards using conventional resins, thus significantly shortening press cycles. Steam injection pressing also permits the use of high temperature curing resins, which are not typically suitable for use in conventional pressing, and which may be cheaper, safer and/or result in a stronger bonded product. And steam injection permits consolidation and curing of relatively thick composite boards, which either do not properly cure in a conventional press or do not cure quickly enough to provide a cost competitive product. Thus, steam injection is known to speed curing of resins improve product quality and shorten production time for wood composite products, particularly products having thick dimensions.
The benefits and advantages of steam injection can be significantly enhanced by conducting the injection in a sealed press, i.e., a press that isolates the press cavity from the surrounding atmosphere. This can be accomplished by sealing the perimeter of the cavity. Alternatively, the entire press can be isolated in a sealed chamber. A sealed press significantly reduces or eliminates the loss of valuable steam and facilitates the injection of steam into the mat at elevated pressures.
Relative to binders that cure at moderate temperatures, such as urea formaldehyde resin (UF) or isocyanate resin (MDI), phenol formaldehyde resin binders require high temperatures for curing, and consequently require a longer press cycle to effect curing throughout the thickness of a composite board profile. Because press cycle time is considered to be a major factor in determining the economy of manufacture of wood composite products, resins requiring longer press cycle times have been avoided due to the additional time required to cure the resin. It was thought that the longer press cycles necessitated by the high curing temperature of a resin could be counteracted by rapidly heating a fast-curing resin with steam injection, or with pre-heating followed by steam injection to cure the resin. However, rapid heating, either by high pressure steam injection, or by a combination of pre-heating and high pressure steam injection, is known to cause fast-curing resin to pre-cure.
It is known that the use of a slower curing resin prevents pre-cure of the resin in process equipment adapted to treat wood fibers with resin prior to formation of a mat for consolidation. U.S. Pat. No. 5,629,083 to Teodorczyk discloses the formation of composite wood products with a slow curing PF binder to prevent pre-cure in a blowline process for resin application to wood fibers before mat formation.
A journal publication by Ernest W. Hsu titled A Practical Steam Pressing Technology for Wood Composites, Proceedings of the Washington State University International Particleboard/Composite Materials Symposium, Pullman, Wash., Apr. 10, 1991, discloses that high-temperature curing resins, such as phenol formaldehyde resins, can be cured in a reasonable range of press times by steam injection in a sealed press. A conference abstract attributed to Ernest W. Hsu titled Comparison of Fiberboards Bonded with PF and UF Resins (1995), discloses that press times for phenol formaldehyde resin bonded fiberboard can be substantially reduced, and thus can be made comparable to UF-bonded fiberboard, by manipulating fiber mat temperatures, molecular weight distribution of PF resins and pressing parameters.
Pre-heating a wood composite mat is known to reduce press time and to prevent pre-cure of surface layers of the mat in the press cycle. U.S. Pat. No. 3,649,396 to Carlsson discloses preheating of furnish with a steam saturated air stream to a temperature close to the curing temperature of the binder to shorten press time, and to prevent premature curing of mat surface layers in the press. Carlsson also teaches that pre-cure is to be avoided in preheating.
U.S. Pat. No. 5,246,652 to Hsu et al. discloses that good bonding strength of a phenol formaldehyde binder can be achieved by steam injection. The Hsu et al. '652 patent discloses a method for making phenol formaldehyde resin bonded wood composites with improved resistance to biological attack and fire. The Hsu '652 patent does not distinguish between slow and fast curing phenol formaldehyde resins.
Despite the indication by Hsu that good bonding strength of a phenol formaldehyde binder can be achieved by steam injection, and that high-temperature curing resins can be cured in a reasonable range of press times by steam injection, the use of phenol formaldehyde resins in steam pressing has been found to be generally unsatisfactory, particularly in commercial applications. The generally unsatisfactory results are attributed to low or inconsistent internal bond strength of the consolidated product (see Lim et al. in U.S. Pat. No. 5,217,665).
As noted above, phenol formaldehyde resins are significantly less expensive to use. Thus, there is a need for a method for making composite board products using phenol formaldehyde resin in a reasonable press time such that the products consistently have suitable properties, such as, for example, high internal bond strength, dimensional stability, durability, etc.